This invention relates generally to the monitoring and treatment of a human or animal subject in cardiac arrest from the electrocardiogram (ECG) of the subject's heart. More particularly, the invention is related to determining clinically useful parameters from the subject's ECG which help guide therapeutic interventions during ventricular fibrillation. As used herein, ventricular fibrillation is intended to include asystole which may be considered a form of ventricular fibrillation.
Each year more than 350,000 sudden cardiac deaths occur in the United States. The successful development and implementation of emergency medical services has resulted in saving many of these subjects. Training the public to perform basic lifesaving maneuvers, like cardiopulmonary resuscitation (CPR), as well as providing easy and early access to medical help, which provides skilled rescuers at the subject's location within minutes of the arrest, has saved lives. However, survival from out-of-hospital cardiac arrest is much lower than theoretically possible. Even the advent of automatic and semi-automatic external defibrillators, and their widespread use by first responders to victims of sudden cardiac arrest, has had only a modest impact on survival.
Although electrical countershock is the most effective treatment of ventricular fibrillation, there is biochemical, histological, and clinical evidence to suggest that electrical countershock can cause myocardial injury. When the duration of ventricular fibrillation is prolonged and the heart is not metabolically conducive to countershock administration, the cumulative energy applied from unsuccessful countershocks to the fibrillating myocardium may impair subsequent efforts at successfully converting the heart to a pulsatile rhythm, that is a cardiac rhythm that allows effective perfusion to the subject. Thus, after more prolonged durations of ventricular fibrillation, therapy aimed at improving myocardial perfusion and, thus, the metabolic state of the myocardium, prior to countershock administration, appears to optimize outcome.
In our U.S. Pat. No. 5,077,667 entitled MEASUREMENT OF THE APPROXIMATE ELAPSED TIME OF VENTRICULAR FIBRILLATION AND MONITORING THE RESPONSE OF THE HEART TO THERAPY, we disclosed a technique for accurately estimating the elapsed time of a subject in ventricular fibrillation. In our patent, time domain samples of the subject's electrocardiogram (ECG) signal are transformed to a frequency domain spectrum and the median frequency, which bisects the energy of the power spectrum, is detected. The median frequency is compared with a pattern of experimentally obtained median frequency data to estimate the elapsed time of ventricular fibrillation. This information may be used to establish the most appropriate time to countershock a subject.
Estimation of the elapsed time of ventricular fibrillation can be affected by many factors. These include whether the subject received cardiopulmonary resuscitation prior to the time that the measurement was made and the degree of perfusion of the heart. For example, the subject may be in cardiac arrest clinically but still have some degree of myocardial perfusion when the heart is in a rhythm called "ventricular tachycardia" or "Torsades de Pointes." Thus, when cardiopulmonary resuscitation generates adequate myocardial perfusion, or if the subject is in a condition with some degree of myocardial perfusion, the metabolic state of the myocardium may not deteriorate as rapidly, despite the fact that the subject is in cardiac arrest clinically. Accordingly, it is desirable to provide a more accurate determination of the metabolic state of the myocardium which takes into account the level of myocardial perfusion prior to measurement of the metabolic state of the myocardium. It is additionally desirable to more accurately predict whether attempts to countershock the subject will result in conversion of the heart to an organized, pulsatile rhythm, in order to avoid the application of unnecessary and potentially harmful countershocks. It is additionally desirable to provide a non-invasive method of guiding therapeutic interventions during ventricular fibrillation and asystole.